Extracting instructive meaning from the juxtaposition of two verses is universally acceptable as a method of interpretation only in D'varim. The example used by the g'mara to prove this includes one of the aforementioned p'sukim. In this issue of Yeshivat Har Etzion's Daf Kesher, the topic is addressed. Its mention is attributed to the Sefer Hamanhig, on whom the Mishna B'rura asks exactly @jake's question in the comment below: How could it be that a mitzva from the Torah is pushed off beyond the age of bar mitzva?

It is suggested that although an interesting symbolic connection exists between wrapping oneself in a talis and protecting one's wife or imitating God's ways, citing the pasuk is intended to be a postfix hint and not an ab initio source.

(Also see this Satmar publication for extensive treatment of the same question: פרי תמרים)

"Extracting instructive meaning from the juxtaposition of two verses" is only acceptable as a method of rabbinic midrash halacha. Never before have I heard of such a method being used to provide for "giving rise to a minhag", especially one that counters the general idea of one being obligated in mitzvos from bar mitzva age.
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jakeMay 22 '11 at 23:05

Re "citing the pasuk is intended to be a postfix hint and not an ab initio source." Great way of describing derech asmachta. Also, the article in P'ri T'marim, in my opinion, does a better job of justifying the minhag. I would recommend summarizing its conclusion as part of your answer.
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jakeMay 22 '11 at 23:42

In minhag Ashkenaz, paseukim are frequently darshened in ways not found in Chazal in order to justify minhagim. Aside from tzitzit, another example is the minhag of not praying ma'ariv of shavuot before tzait hakochavim. This comes from the paseuk of sefira being "seven complete weeks". However the first person to justify this minhag was R' Avraham Horowitz, the father of the Shelah HaKadosh, which means this justification significantly postdates chazal.
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ChanochMay 23 '11 at 13:27

The B'nei Yissachar says that kabalistically, a Talit Gadol draws down an Or Makif (Transcendent Light). This can only be drawn down when a person is joyful (with Simcha Shel Mitzvah). Since the Gemara says (Yevamot 62B) "A man without a wife lives without joy", and (Shabbos 152A) "The joy of the heart is a woman" an unmarried person does not wear a Talit Gadol.

Welcome to Judaism.SE, and thanks very much for bringing in this idea!
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Isaac MosesMay 25 '11 at 18:27

1

The spouse serving as a •Makif*, an internalizer of the holiness of the Neshama to the righteous, scholarly person, is referred to in Chapter 8 (The Five Parts of the Soul) of the holy book Flames of Faith by Rabbi Zev Reichman, on page 82.
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Adam MoshehJul 5 '12 at 14:38

There is no chiyuv to wear Tzitzis unless you are wearing a 4-cornered garment. That used to be what they wore in bavel, but over time the style of clothing changed. It became customary to specifically put on a 4-cornered garment so as to get the mitzvah of Tzitzis. It seems that the custom to wear a large 4-cornered garment was only adopted by married people, perhaps due to financial reasons.

Note that German-Jews do wear Tallesim before marriage, and I think R'Hershel Schachter also thinks its a good idea.